In process industry, the consistency of pulp can be measured using a rotating consistency transmitter that has two shafts within each other. A drive shaft is driven by a motor and a measuring shaft is flexibly mounted with bearings on the drive shaft, which makes an inter-shaft swivel possible. The swivel, which can also be indicated as an offset, may be a few degrees at most. There may be projections at the end of the measuring shaft, and the pulp to be measured tries to slow down the rotation of the projections with its consistency. The slowing down force depends on the consistency and causes a torque between the shafts, which tries to increase the offset between the flexibly bearing-mounted shafts. However, the shafts are electromagnetically coupled, and the offset between the shafts is kept constant by adjusting the amount of current supplied to the coils, whereby the magnetic forces compensate for the torque between the shafts.
The swivel between the shafts can be measured with an optocoupler that comprises an optical transmitter and receiver in such a manner that a toothed wheel rotating along with the shafts is attached to each shaft, and its teeth act as choppers of the beam between the optical transmitter and receiver, thus forming a pulsed signal to the receiver. When the shafts are in-phase, the teeth of the toothed wheels may converge. But when offset forms between the shafts, the teeth of the wheels shift correspondingly with respect to each other. This alters the length and ratio of the pulses of the optical signal. A measured pulse ratio is ordinarily further converted into a mean direct voltage. The offset between the shafts is thus directly proportional to the direct voltage that, in turn, is proportional to the pulse ratio.
There are, however, problems related to his type of measurement. The temperature, temperature changes and aging of measuring electronics affect the measuring results, and the error caused by them cannot be eliminated from the measurement. In addition, changes in the rotation speed of the shafts may cause errors.